Field of the Disclosure
The present invention relates generally to power converters, and more specifically to transformers that can reduce the effects of electromagnetic interference (EMI).
Background
Electronic devices use power to operate. Switched mode power converters are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a high voltage alternating current. In a switch mode power converter, a high voltage alternating current (ac) input is converted to provide a well-regulated direct current (dc) output through an energy transfer element. One example of a switch mode power converter is a flyback converter, which uses a transformer to transfer energy from an input winding to an output winding.
During the normal operation of the flyback converter, the voltages across both input and output windings of the transformer transition in accordance with the standard flyback power supply operation. These transitions generate displacement currents in the electrical earth through various parasitic capacitances. These displacement currents are detected as common mode noise and are measured by a piece of test equipment called a Line Input Stabilization Network (LISN).
One way to reduce common mode noise is the use of a Y-capacitor. The Y-capacitor provides a low impedance path for displacement currents flowing between the input and output windings of the transformer to return to their source without flowing through the electrical earth. The currents of the Y-capacitor are not detected by the LISN and therefore act to reduce common mode noise.
However, a power supply design may limit the Y-capacitor to a small value or no Y-capacitor at all. One way to reduce common mode noise with a smaller Y-capacitor is through the use of one or more additional windings that can reduce the capacitive displacement current between the transformer input winding and output winding by balancing the relative electrostatic fields generated by all windings with the transformer relative electrical earth through the selection of the physical position and number of turns in additional windings. However, the effectiveness of the additional windings can be limited due to the mechanical constraints such as bobbin size, or the number of wound turns due to the wire gauge.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.